A structural defect identification approach based on a continuum damage model

This paper introduces a structural identification technique built on finite element (FE) model updating. The FE model is parameterized by a structural parameter that continuously describes the damage in the structure, and besides, an evolution equation of this damage parameter is presented. The model updating is accomplished by determining the subset of this damage parameters that minimizes a global error derived from the dynamic residue vectors, which is obtained by introducing the experimental modal properties into the original model eigenproblem. A mode-shape projection technique is used in order to achieve compatibility between the dimension of the experimental and analytical models. The adjusted model maintains basic properties of the analytical model as the sparsity and the symmetry, which plays an important role in model updating-based damage identification. The verification and assessment of the current structural defect identification is performed on a analytically derived bidimensional truss structure and on a cantilever bidimensional Euler–Bernouilli beam through a virtual test simulator. This simulator is used to realistically simulate the corrupting effects of noise, filtering, digital sampling and truncation of the modal spectrum. The eigensystem realization algorithm along with the common-based normalized system identification were utilized to obtain the required natural frequencies and mode shapes.     

An efficient model of mixed-mode delamination in laminated composites including bridging mechanisms

A refined model is developed to analyse delamination in composite laminates accounting for bridging stresses at crack faces. The analysis adopts a first-order shear deformable layer-wise kinematics for the laminate and an interface model simulating mixed-mode fracture in the presence of a bridged delamination. A penalised interface simulates adhesion between layers and provides energy release rates through its strain energy density while a two-parameter softening interface with a limit displacement models bridging stresses. Delamination evolution analysis is performed by solving the non-linear boundary value problem resulting from a stress analysis coupled with opportune propagation conditions. Numerical examples are presented for composite laminates subjected to both pure mode and mixed-mode loading conditions and the results are compared with those obtained adopting classic delamination models. Analytical formulae for energy release rate evaluation are also proposed to carry out an investigation of the main factors governing accuracy in predicting delamination growth. The proposed approach captures important effects which are not included in classic delamination models. The accuracy of the model is assessed by comparisons with 2D finite element results obtained by using delamination interface elements. The finite element model agrees well with results obtained by using the proposed approach.     

Coupled continuum damage mechanics and crystal plasticity model and its application in damage evolution in polycrystalline aggregates

In the present study, damage initiation and growth in a polycrystalline aggregate are investigated. In this regard, an anisotropic continuum damage mechanics coupled with rate-dependent crystal plasticity theory is employed. Using a thermodynamically consistent procedure, a finite deformation formulation is derived. For this purpose, the damage tensor is incorporated in the crystal plasticity formulation for a cubic single crystal. The damage evolution is considered to be dependent on the history of damage, equivalent plastic strain, stress triaxiality, and Lode parameters. This material model is implemented in the commercial finite-element code Abaqus/Standard by developing a user material subroutine (UMAT). Using the available experimental tests of 316L single crystal in the literature, the crystal plasticity hardening and damage parameters are calibrated considering the stress–strain curve before and after necking, respectively. The damage sites in a single-phase polycrystalline aggregate are also considered using a polycrystalline model consisting of grains with random sizes and orientations. The results show that the damage arises at the grain boundaries and triple junctions. Moreover, growth of the damage mostly occurs in the grains with higher Schmid factor compared to the neighboring grains. The presented model manifests capacity for determination of damage initiation sites and damage evolution in polycrystalline models.

     

Three-dimensional finite-element analysis of laminated composites

A three-dimensional finite-element analysis treating the mechanical response of thick laminated composite plates in bending is presented. An isoparametric solid element with a cubic displacement expansion in planform and a linear variation through the thickness is used to model each layer of the laminate. The degrees-of-freedom of the element are retained at its boundaries so that interconnections between lamina with different fiber orientations can be made at their interfaces. An incore version of the conjugate gradient technique, which does not have bandwidth restrictions, is used to minimize the total potential energy of the system with the number of iterations to convergence being about one-fifth the total global degrees-of-freedom. Because a three-dimensional analysis is used, the effects of thickness-stretching, transverse shear, extension, and bending deformations are obtained. Comparisons with three-dimensional elasticity solutions are in excellent agreement and show the necessity of having individual elements for each layer when they have different fiber orientations and when the plates are thick.     

Use of an artificial neuroadaptive robot model to describe adaptiveand learning motor mechanisms in the central nervous system

Based on previous physiological information, this paper proposes a model of cerebellum motor learning based on a neuroadaptive robot manipulator controller. Compliance (or impedance) control is chosen as the basis of the model in preference to alternative robot control strategies because muscles do not act like pure force generators such as torque motors nor as pure displacement devices such as stepper motors but instead act more like tunable springs or compliance devices. Compliance control has the further advantage that it is applicable for a variety of motor tasks, and is both more robust and simple than alternative control strategies. Simulation results are presented to verify the performance of the proposed model. Specific results are presented for the applications of impedance control to the case where the end-effector is interacting with surfaces. By setting the equilibrium position of the end-effector beyond the obstacle (wall), it can be assured that the end-effector will touch the surface rather than crush it. The power of the phase spare to analyze the behavior of the system during movement is demonstrated.     

Thermally induced buckling of laminated composites by a layerwise theory

Thermal buckling analysis of laminated composites is presented by using a layer-wise theory. Governing buckling equations are derived from the variational principle and a finite element method is developed to formulate the problem. Numerical results are obtained and compared with those of other theories addressing the effects of the thickness-to-span ratio, lamination angle, the ratio of thermal expansion coefficients and degree of orthotropy on buckling temperature for antisymmetric angle-ply laminates. It is found that a layer-wise approach may be necessary for more accurate thermal buckling analysis of laminated composites.     

An elasto-plastic damage model cast in a co-rotational kinematic framework for large deformation analysis of laminated composite shells

This paper presents an elasto-plastic damage model that is based on irreversible thermodynamics and internal state variable formalism for the analysis of multi-layered composites. The model is based on a damage surface that is defined in terms of an internal damage variable of energy, along with a set of rate-independent elasto-plastic constitutive equations defined in an effective stress–strain space. Employing the operator splitting methodology, a three-step predictor/multi-corrector algorithm is developed that includes an elastic predictor, a plastic corrector, and a damage corrector. The constitutive model is cast in a co-rotational kinematic framework for damage analysis in laminated plates and shells undergoing large deflections. Numerical examples are presented to demonstrate the accuracy and range of applicability of the proposed model.     

Development of a geospatial model to quantify, describe and map urban growth

In the United States, there is widespread concern about understanding and curbing urban sprawl, which has been cited for its negative impacts on natural resources, economic health, and community character. There is not, however, a universally accepted definition of urban sprawl. It has been described using quantitative measures, qualitative terms, attitudinal explanations, and landscape patterns. To help local, regional and state land use planners better understand and address the issues attributed to sprawl, researchers at NASA's Northeast Regional Earth Science Applications Center (RESAC) at The University of Connecticut have developed an urban growth model. The model, which is based on land cover derived from remotely sensed satellite imagery, determines the geographic extent, patterns, and classes of urban growth over time.Input data to the urban growth model consist of two dates of satellite-derived land cover data that are converted, based on user-defined reclassification options, to just three classes: developed, non-developed, and water. The model identifies three classes of undeveloped land as well as developed land for both dates based on neighborhood information. These two images are used to create a change map that provides more detail than a traditional change analysis by utilizing the classes of non-developed land and including contextual information. The change map becomes the input for the urban growth analysis where five classes of growth are identified: infill, expansion, isolated, linear branch, and clustered branch.The output urban growth map is a powerful visual and quantitative assessment of the kinds of urban growth that have occurred across a landscape. Urban growth further can be characterized using a temporal sequence of urban growth maps to illustrate urban growth dynamics. Beyond analysis, the ability of remote sensing-based information to show changes to a community's landscape, at different geographic scales and over time, is a new and unique resource for local land use decision makers as they plan the future of their communities.     

Design optimization of laminated composites using a new variant of simulated annealing

The aim of this study is to minimize the thickness (or weight) of laminated composite plates subject to both in-plane and out-of-plane loading. A new variant of the simulated annealing algorithm is proposed to optimize the lay-up design. Fiber orientation angle and number of plies in each lamina are used as design variables. Considering static failure as the critical failure mode, the maximum stress and Tsai–Wu criteria are used together to predict failure. Numerical results show that the optimization methodology proposed in this study can find the globally optimum laminate designs even with a high number of design variables.     

A hybrid model for rate-dependent hysteresis in piezoelectric actuators

A hybrid model is proposed in this paper to describe the static nonlinear and dynamic characteristics of rate-dependent hysteresis in piezoelectric actuators. In this model, a neural network based submodel is implemented to approximate the static nonlinear characteristic of the hysteresis while a submodel with the first-order differential operators is constructed to describe the dynamic behavior of the hysteresis. In this paper, a special hysteretic operator is proposed to extract the hysteretic feature of the hysteresis. Then, an expanded input space with such special hysteretic operator is constructed. Based on the constructed expanded input space, the neural network can be implemented to approximate the hysteresis phenomenon. The submodel to describe the dynamics is a sum of the weighted first-order differential operators. Finally, the experimental results of applying the proposed method to the modeling of hysteresis in a piezoelectric actuator are also presented.     

A population dynamics model to describe gene frequencies in evolutionary algorithms

The performance of evolutionary algorithms (EAs) may heavily depend severely on a suitable choice of parameters such as mutation and crossover rates. Several methods to adjust those parameters have been developed in order to enhance EA performance. For this purpose, it is important to understand the EA dynamics, i.e., to appreciate the behavior of the population. Hence, this paper presents a new model of population dynamics to describe and predict the diversity in any particular generation. The formulation is based on selecting the probability density function of each individual. The population dynamics proposed is modeled for a generational population. The model was tested in several case studies of different population sizes. The results suggest that the prediction error decreases as the population size increases.     

Numerical analysis of anisotropic ductile continuum damage

The paper deals with the numerical analysis of large elastic–plastic deformation behavior of anisotropically damaged ductile solids based on a generalized macroscopic theory within the framework of nonlinear continuum damage mechanics. Estimates of the stress and strain histories are obtained from a straightforward numerical integration algorithm based on operator split methodology which employs an inelastic (damage–plastic) predictor followed by an elastic corrector step. The finite element method is used to approximate the linearized variational problem. Furthermore, identification of material parameters is discussed. Numerical simulation of the elastic–plastic deformation behavior of damaged tension specimens demonstrate the efficiency of the formulation.     

Formulation of a nonlinear compatible finite element for the analysis of laminated composites

Increased use of laminated composites has pointed out the need for better analytic tools. These tools must be able to correctly account for normal shear in the laminates and should be able to solve nonlinear problems. The finite element method is applied in this paper to analyze laminated composites using an approach which includes both nonlinear and normal shear effects. A finite element is formulated from basic elasticity equations. The most unique characteristic of the element is the manner in which normal shear is handled. The normal to a reference surface is allowed to not only rotate but also to change shape. Not only is displacement continuity imposed at lamina interfaces, but also slope continuity is guaranteed. After a complete general formulation of the finite element is presented, the method is specialized to plates so that the convergence characteristics, the accuracy and the applicability of the element can be studied. The finite element does an excellent job of analyzing laminated composites, and gives the analyst the ability to do nonlinear analysis of laminated composites under a variety of loading and boundary conditions.     

A hybrid PSO-GA algorithm for optimization of laminated composites

This paper presents a new hybrid Particle Swarm Algorithm (PSO) for optimization of laminated composite structures. The method combines the standard PSO heuristics with Genetic Algorithm operators in order to improve the algorithm performance. Thus, operations that are important to the optimization of laminated composites such as mutation and layer swap are incorporated into the method. A specially designed encoding scheme is used to represent the laminate variables and the associated velocities. A study is carried-out to select the best variant of the proposed method for the optimization of laminated composites, considering different swarm topologies and genetic operators. Both strength maximization and weight minimization problems are considered. A meta-optimization procedure is used to tune the parameters of each variant in order to avoid biased results. The results showed that the proposed method led to excellent results for both traditional and dispersed laminates, representing a significant improvement over the standard PSO algorithm.     

How to describe or design a polyhedron

Polyhedra are basic three-dimensional objects, appearing at many levels in the algorithms of computational geometry, computer-aided design, computer vision, and robotics. What data can we use to describe a polyhedron in 3-space? What independent choices can we make when constructing a polyhedron? These are two aspects of a single question investigated in this article. The answers depend both on the level of geometry we are using (Euclidean, similarity, projective, combinatorial) and on the source of the geometric data. The constructions and representations are translated from classical and modern geometric practice. Classical theorems and techniques of Cauchy, Steinitz, Maxwell, Minkowski, and Alexandrov are transferred to this setting. Other recent geometric results are also described and a number of unsolved problems are raised.     

On the elastic modulus degradation in continuum damage mechanics

To measure accurately the elastic modulus of a metal, E, can be a difficult task when a specimen undergoes plastic strains. Moreover, some failure criteria, such as those associated with Continuum Damage Mechanics, require the change of elastic modulus with strain to define a measure of damage, D, in a material or structure. Thus, it is important to assess the possible geometrical influence of a specimen on the measurement of the elastic modulus at different deformation levels. It is shown in this article, with the aid of a numerical simulation, that any plastic strains induce important geometrical effects in the evaluation of E, which have a significant influence on the evaluation of the scalar damage parameter, D.     

Finite element computations of anisotropic continuum damage in reinforced concrete

A coupled damage-plasticity concept for the modeling of reinforced concrete structures is presented. Concrete is considered in the framework of mixture theory as a composite consisting of cement matrix and aggregate. Anisotropic damage in the cement matrix and plasticity at the contact interface between cement matrix and aggregate describe the nonlinear constitutive behavior of concrete. Reinforcement is taken into account with three-dimensional rebar-elements. Damage at the interface between concrete and reinforcement is described within the framework of a microplane theory. The proposed concept is verified for some reinforced concrete structures.     

A Formalism to describe cyclogram testing models and perform model verification

We develop a formal system for describing cyclograms in interval temporal logic. We show that it can be applied for modeling and verification (testing correctness) of cyclogram models (CM). We describe how a rule-based knowledge base of a CM and instruments for constructing algorithms that output discrete controlling influences can be used to improve testing quality.     

Finite element analysis of damping the vibrations of laminated composites

There are several ways of decreasing the vibration energy of structures, such as diminishing the source energy, designing structures with desired eigen-frequencies and excitation frequencies, using materials that have damping properties, etc. Special damping layers made of various viscoelastic materials are widely applied in structures subjected to dynamic loading, especially those used in ship building and aerospace technology. A typical structure is that whose basic layer is covered with a damping layer and a thin constraining layer. Such classical sandwich structures have been widely investigated, especially with reference to the analysis of elastic vibrations.